This invention relates to optical systems, and in particular to optical systems used in relation to conformal windows in aircraft sensor systems.
An optical sensor receives radiated energy from a scene and converts it to an electrical signal. The electrical signal is provided to a display or further processed for pattern recognition or the like. Optical sensors are available in a variety of types and for wavelengths ranging from the ultraviolet, through the visible, and into the infrared. Optical sensors are used in both commercial and military applications. In some applications the optical sensor is fixed in orientation, and in others the optical sensor is movable such as by a pivoting motion or a pivoting and rolling motion to allow sensing over a wide angular range.
The optical sensors generally employ a photosensitive material that faces the scene and produces an electrical output responsive to the incident energy. The photosensitive material and remainder of the sensor structure are rather fragile, and are easily damaged by dirt, erosion, chemicals, or high wind velocity. The sensor is placed behind a window through which it views the scene and which protects the sensor from such external effects. The window must be transparent to the radiation of the operating wavelength of the sensor and resist attack from the external forces. The window must also permit the sensor to view the scene over the specified field of regard. This field of regard is the angular extent over which the sensor must be able to be pointed to view the scene. The field of regard may extend over wide angles and in two rotational directions. For example, a look-down sensor on a high-speed aircraft must have a field of regard that extends over large specified angles from front-to-back (elevational angle) and from side-to-side (azimuthal angle).
The window would ideally introduce minimal wavefront distortion of the scene over the field of regard of the sensor, particularly if the sensor is an imaging sensor. The larger and thicker the window, the more likely is the introduction of significant wavefront distortion. Where there is wavefront distortion, as is always the case to some degree, it is desirable that such wavefront distortion be of a predictable, regular type that may be compensated for, at least in part, with an optical corrector placed between the window and the sensor or by electrical circuitry or processing.
A wide variety of sensor windows have been used in various aircraft applications. In many cases such as low-speed helicopters, flat windows are acceptable. Windows that are shaped as segments of spheres or conic sections are used in aircraft (including missile) applications, but for these windows the radar signature and the aerodynamic drag of the window are large.
For applications involving aircraft (including missiles) operating at high speeds, the window should be relatively aerodynamic such that the presence of the window extending into the airstream does not introduce unacceptably high and/or asymmetric aerodynamic drag to the vehicle. A conformal window of this type typically has a ratio of the length of the window to the diameter of the window, termed the xe2x80x9cfineness ratioxe2x80x9d in the art, of greater than about 0.5. A conformal window is beneficial to reducing drag and increasing the range of the aircraft. However, existing conformal windows introduce large wavefront distortions into the sensor beam, particularly for high azimuthal pointing angles of the sensor.
It is known to form the window with a torus shape that provides a compromise between good aerodynamic properties and minimal wavefront distortion. This approach, described in U.S. Pat. No. 5,914,821, is effective for a number of applications. In other applications, even better aerodynamic performance in terms of extended range and speed is required than possible with this approach.
There is a need for an improved optical system to be used in demanding applications such as look-down and side-looking windows in high-speed aircraft. The present invention fulfills this need, and further provides related advantages.
The present invention provides an optical system that is suitable for use in high-speed aircraft. The optical system includes a window, a sensor, and an optical corrector positioned between the sensor and the window. The window is conformal and introduces minimal drag in the airflow, improving the range and speed of the aircraft. The optical corrector corrects wavefront distortion introduced into the optical path of the sensor by the window. The optical corrector may be made relatively light in weight. The sensor may be sensitive to any of a wide variety of types and wavelengths of energy.
In accordance with the invention, an optical system comprises a window, and an optical corrector comprising a curved piece of a transparent material having a front surface and a back surface. At least one of the front surface and the back surface of the optical corrector has a torus shape defined in Cartesian coordinates by the relation
Z(x,y)=Zprev(x,y)+Lx(x,y)+Ly(y).
In this relation,
Zprev(x,y)=cAxcfx812/[1+(1xe2x88x92(kA+1)cA2xcfx812)xc2xd]+dAxcfx814+eAxcfx816+fAxcfx818+gAxcfx8110,
xcfx812=x2+y2,
Ly(y)=C1y2/[1+(1xe2x88x92(C2+1)C12y2)xc2xd]+C3y4+C4y6+C5y8+C6y10+C7y12+C8y14,
Lx(x,y)=Cxx2/[1+(1xe2x88x92(kx+1)Cx2x2)xc2xd]+C13x4+C14x6+C15x8+C16x10+C17x12+C18x14,
Cx=C11+C21y+C31y2,
and
kx=C12+C22y+C32y2.
wherein cA, kA, dA, eA, fA, gA, C1-C8, C11-C18, C21-C22, and C31-C32 are constants. A sensor system, comprising a sensor, is positioned such that the optical corrector is between the window and the sensor.
Here, Zprev(x,y) is a generalized aspheric shape of the typical optical surface. Ly(y) is the parent profile of the torus shape. Lx(x,y) is the profile parallel to the x-z plane sweeping along the parent profile. Cx and C1 are the curvatures of the profiles Lx(x,y) and Ly(y), respectively.
The values of cA, kA, dA, eA, fA, gA, C1-C8, C11-C18, C21-C22, and C31-C32 are constants for any selected optical corrector and define the shape of the optical corrector at any selected location (x, y, z) according to the relations set forth above. The coefficients C3-C8 and C13-C18 are higher order coefficients similar to a typical aspheric surface shape described by Zprev(x,y). These coefficients modify the basic surface shape slightly to obtain better wavefront uniformity. There are no known limitations on the values over which the constants may range, except as noted.
The optical system of the invention allows the window to be optimally shaped for aerodynamic performance. The wavefront distortion which the window introduces into the optical path is corrected with the optimally shaped optical corrector. This combination permits the aircraft designer to optimize the window to achieve the best aerodynamic performance and to independently optimize the optical corrector to achieve the required optical performance to produce good-quality images. By contrast, where the window has a particular curvature to minimize wavefront distortion, as in some prior approaches, either the aerodynamic performance or the optical performance is compromised to some degree. That compromise in performance is not experienced in the approach of the invention.
The optical corrector desirably eliminates wavefront distortion over a wide field of regard, but any residual wavefront distortion may be further reduced or removed by electronic signal processing.
In many instances, the window preferably is an aerodynamic conformal window with a fineness ratio of greater than about 0.5. In one embodiment, the window has an outer surface with a first ellipsoidal shape, and an inner surface with a second ellipsoidal shape.
The optical corrector is preferably in the form of a curved strip of transparent material mounted in the view path of the sensor. This strip form of optical corrector is usually mounted on a gimbal structure, such as a roll gimbal. The optical corrector thus comprises a curved strip of a transparent material having a front surface and a back surface, with at least one of the front surface and the back surface having a shape which is a segment of a torus shape. The strip form of the optical corrector subtends an azimuthal arc as large as necessary to cover the azimuthal field of regard required for the sensor system, typically on the order of about 40 degrees. The strip optical corrector may be relatively narrow in angular extent, usually less than about 10 degrees and typically about 3 degrees, in the orthogonal direction, as it is rotated on the roll gimbal to achieve 360 degrees of coverage about its centerline.
The sensor may be of any operable type. It may be sensitive to radiation in the ultraviolet, visible, or infrared ranges, or to specific bands within these ranges. Such sensors are known in the art. The window and optical corrector are transparent to the same wavelength as is sensed by the sensor. The sensor system, which usually includes a telescope that directs the optical beam into the sensor, is typically mounted on a gimbal such as a roll/nod gimbal.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.